Depends what your target noise margin is.
If the noise margin/SNR immediately after a re-sync is 9, 12 or 15db, then you've been subjected to an increase in the target margin, to increase stability.
If you stay connected (without any re-syncs at all) for more than 14 days, the system should, but probably won't, drop the target margin by 3db.
So, you should eventually get your speed back.
Never happened to me, I needed manual intervention.
If yours doesn't go back on it's own, you can ask your ISP to have your line re-trained, or your target noise margin reduced to 6db, but I doubt if it'll work.
The only sure way to get it fixed, is by regrading to a product that will force a line re-train (such as back to fixed speed, or from a home to office product).

Is there any way I can determine the required noise margin?

When I rebooted the router yesterday afternoon it was at 7.5 and dropped from something like 3.6meg to 3.2 meg, whereas before if I had been at say 3.2 meg and rebooted with a 6db SNR it rose to between 3.5 and 3.9.

The frustrating bit is that for the most part, it lingered at around 3.8 to 3.9meg in a relatively stable manner and I would like to get back to that.

Well I think the only option is for me to ask for a retrain or drop in target margin as from what I gather, Demon are not the most flexible when regrading products, especially in the first 12 months.

Is there an easy way to determine my target margin?

To be able to tell, you need to know the minimum SNR margin that your router will reliably operate at, then add the number of dB's fluctuation (i.e. if the minimum SNR is 2dB and your line fluctuates by 4dB, then the ideal setting will be 6dB). Unfortunately every make/model of modem has a minimum level, so generally it's a case of "suck it and see".

Is there an easy way to determine my target margin?

Based on the assumption your noise margin shouldn't shift too much, too quickly, the best way is to initiate a manual re-sync, and be very quick in checking the router web-page.
Check the noise margin straight away, and take a calculated guess.
The target margins go in 3db steps (the normal default being 6db), so if you end up with something like 5 or 7db (for example), there's a fair chance your target margin is 6db.
If you're still on a 6db target margin, you need to look elsewhere for the cause of the drop in sync speed.
Here is a good place to start.

Ok, yesterday I dropped my PPP then rebooted the modem in the afternoon and managed to get up to 3.4meg sync.

So this afternoon I thought I would give it a go again and it dropped to 3.3 so I tried again and it went right up to 3.8.

Unfortunately my up sync has now gone from around 800 to 320 LOL.

One step forward six steps back

Any ideas? I know I can handle 4meg down and 800 odd k up.